Expanded design and operational requirements for turbomachinery can push the system, subsystem, and component designs against the limits of their fatigue boundaries. This in turn raises the design complexity needed for expanded performance as well as the vibration sensitivity of the blade rows due to the expanded mission roles. The design trends for fans and compressors are seen in higher relative Mach number and tip speed, lower stage count, higher stage loading, and a lower radius ratio. This leads to thin, twisted, low aspect ratio blades with high steady stress levels, low foreign object damage margin, and stronger aerodynamic forcing functions. The end result is an integrally bladed rotor with low damping and low margin. The same trends are seen in turbines, with the added complexities of higher inlet temperatures, warmer cooling air, and more complex cooling schemes. With reduced margins, more detailed characterization of current and potential turbomachinery systems is required.
The disclosed embodiment of the present invention is an innovative blade health monitoring system capable of dramatically improved classification of blade vibration response in terms of mistuning and closely spaced modes. This is needed because of limitations in the prior art for measuring blade response during operation. Strain gage surveys can provide useful data to engineering teams, but are generally limited by several factors. First, the time and cost of strain gage testing is prohibitive for most circumstances. Strain gages have limited fatigue and durability, generally reducing the scope of planned vibration surveys. A limit to the number of available telemetry (or slip ring) channels also means that a small number of blades can be monitored at a given time.
For these reasons, much of the prior art involves some form of Non-contacting Stress Measurement System (NSMS), also known as tip-timing. However, there are several technical challenges in NSMS that have limited the applicability of this technology for general purpose blade health monitoring. Two of the most pressing are the undersampling that is inherent in time-of-arrival data processing and the uncertainty that is introduced by inferring, as opposed to calculating, the mode of vibration. Incorrect mode inferences are known to lead to order of magnitude errors in blade stress estimates.
Significant advancements have been made in NSMS capability by utilizing multiple blade tip sensors at predefined circumferential and axial locations. While this has somewhat reduced the uncertainty with identification of the vibration mode, the spatial resolution of such a system is limited to the deflection at the blade tip. Using a large number of blade tip sensors has also reduced the undersampling error, but this works against the real-world limits on the size, weight, and reliability of the measurement system as a whole. The use of multiple blade tip sensors is also strongly dependent on a-priori knowledge of the vibration modes that are present in order to determine the required number of sensors and their optimal locations. This dependency limits the adaptability of the system for general blade vibration surveys and HCF troubleshooting.